JP3105159B2 - ALC circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ALC (Auto
o Level Control) circuit.
The present invention relates to an ALC circuit used for a radar device or the like.

[0002]

2. Description of the Related Art FIG. 12 shows an automatic gain control device (Auto Gain) conventionally used in a receiver of a radar device.
Control device: hereinafter referred to as an AGC device). The AGC apparatus includes a variable attenuator 20 for changing a gain of a receiver, a logarithmic conversion circuit 21 for converting a received signal into a logarithmic value, and a subtraction circuit 22 for subtracting a gain setting value from an output value of the logarithmic conversion circuit 21. An addition circuit 23 for adding the previous gain correction amount (attenuation setting amount of the variable attenuator 20) to the output value of the subtraction circuit 22, a hold circuit 24 for holding the gain correction amount, and a transmission pulse width and a transmission frequency number. And a setting value selection circuit 25 for selecting one of the gain setting values 26a to 26c.

Next, the operation of the AGC apparatus will be described. During the gain correction period, the AGC pilot signal is input to the variable attenuator 20 in accordance with the AGC timing signal instead of the received signal. After the pilot signal passed through the variable attenuator 20 is converted into a video signal, the logarithmic conversion circuit 2
It is converted to a logarithmic value at 1. The converted logarithmic value is subtracted by a subtraction circuit 22 from a gain setting value selected by a transmission pulse width and a transmission frequency.

[0004] The output value of the subtraction circuit 22 is added to the previous gain setting value by the addition circuit 23. The value after the addition is the attenuation amount (gain correction amount) of the variable attenuator 20. This control amount is held by the hold circuit 24 until the next AGC timing signal is input.

[0005]

A conventional AGC device is:
With the above configuration, it is not possible to compensate for the gain variation in the AGC device due to the temperature, and if there are many types of frequency numbers and pulse widths, the required number of gain setting values increases and the circuit scale becomes large. growing.

A subtraction circuit, an addition circuit, a hold circuit,
There is a problem that the correction accuracy is reduced due to the error of the variable attenuator.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and can correct the fluctuation in the apparatus due to temperature, requires only one set value, and can perform the correction with high accuracy. The aim is to obtain a circuit.

[0008]

To achieve the above object, an ALC circuit according to the present invention distributes a digitally controlled variable attenuating means for attenuating a pilot signal and a pilot signal passed through the digitally controlled variable attenuating means. Distributing means, linear detecting means for converting a pilot signal distributed by the distributing means into a video signal, A / D converting means for converting a video signal output from the linear detecting means into a digital signal, frequency and temperature information Correction means for correcting the digital signal, a logarithmic conversion means for converting the digital signal corrected by the correction means into a logarithmic value, and a difference between the digital signal logarithmically converted by the logarithmic conversion means and a set value. Subtraction means, a digital signal logarithmically converted by the logarithmic conversion means, and a set value. Comparing means for comparing the output of the subtraction means as an initial value UP / DOW an output of the comparing means
And an UP / DOWN counter for setting the amount of attenuation of the digitally controlled variable attenuation means using the count value as the N control signal.

In the ALC circuit according to the present invention, the pilot signal is converted into a video signal by the linear detector,
The compensator corrects the digitally converted video signal based on the temperature and frequency information, and the attenuation of the digitally controlled variable attenuator is corrected and set accordingly. Further, the initial attenuation is given by the subtractor, and thereafter, the UP / DOWN counter corrects the precision with the accuracy equal to or less than the minimum attenuation of the digitally controlled variable attenuator by comparing the level with the set value.

An ALC circuit according to the next invention is a digitally controlled variable attenuating means for attenuating a pilot signal, a distributing means for distributing a pilot signal passed through the digitally controlled variable attenuating means, and a pilot signal distributed by the distributing means. Linear detection means for converting the video signal output from the linear detection means to a digital signal, A / D conversion means for converting the video signal output from the linear detection means to a digital signal, and correction means for correcting the digital signal according to frequency and temperature information;
Storage means for outputting the difference between the digital signal corrected by the correction means and the set value as a logarithmic value; comparison means for comparing the digital signal corrected by the correction means with the set value; output of the storage means And an UP / DOWN counter that sets the output of the comparison means as an UP / DOWN control signal, sets the amount of attenuation of the digitally controlled variable attenuating means using a count value as an initial value.

[0011] In the ALC circuit according to the present invention, the pilot signal is converted into a video signal by the linear detector.
The compensator corrects the digitally converted video signal based on the temperature and frequency information, and the attenuation of the digitally controlled variable attenuator is corrected and set accordingly. Further, the initial attenuation is given by the memory, and thereafter, by comparing the level with the set value, the UP / DOWN counter corrects with an accuracy equal to or less than the minimum attenuation of the digitally controlled variable attenuator.

An ALC circuit according to the next invention comprises a voltage-controlled variable attenuating means for attenuating a pilot signal, a distribution means for distributing a pilot signal having passed through the voltage-controlled variable attenuating means, and a pilot signal distributed by the distribution means. Linear detection means for converting the video signal output from the linear detection means into a digital signal; A / D conversion means for converting the video signal output from the linear detection means into a digital signal; correction means for correcting the digital signal according to frequency and temperature information; Logarithmic conversion means for converting the digital signal corrected by the logarithmic value into a logarithmic value, subtraction means for calculating a difference between the digital signal logarithmically converted by the logarithmic conversion means and a set value,
Comparison means for comparing the digital signal logarithmically converted by the logarithmic conversion means with a set value; an UP / DOWN counter for setting the output of the subtraction means as an initial value and the output of the comparison means as an UP / DOWN control signal; The UP / D
D / A conversion means for converting the output of the OWN counter into a voltage value and setting the attenuation of the voltage control variable attenuation means.

[0013] In the ALC circuit according to the present invention, the pilot signal is converted into a video signal by the linear detector.
The compensator corrects the digitally converted video signal based on the temperature and frequency information, and the attenuation of the voltage-controlled variable attenuator is corrected and set accordingly. Also, the initial attenuation is given by a subtractor, and then U
The output of the P / DOWN counter is converted into a voltage value and corrected with an accuracy equal to or less than the minimum attenuation of the voltage-controlled variable attenuator.

An ALC circuit according to the next invention is a digitally controlled variable attenuating means for attenuating a pilot signal, a distributing means for distributing a pilot signal passing through the digitally controlled variable attenuating means, and a pilot signal distributed by the distributing means. Logarithmic detection means for converting to a logarithmic video signal, A / D conversion means for converting a logarithmic video signal output from the logarithmic detection means to a digital signal, correction means for correcting a digital signal according to frequency and temperature information, Subtracting means for obtaining a difference between the digital signal corrected by the means and the set value; comparing means for comparing the digital signal corrected by the correcting means with the set value; and setting the output of the subtracting means as an initial value. The output of the comparison means is used as an UP / DOWN control signal and the digital control U to set the attenuation of the variable attenuating means
And a P / DOWN counter.

In the ALC circuit according to the present invention, the pilot signal is converted to a logarithmic video signal by the logarithmic detector, and the digitally converted logarithmic video signal is corrected by the corrector on the basis of temperature and frequency information. The attenuation of the variable attenuator is corrected and set. Further, the initial attenuation is given by the subtractor, and thereafter, the UP / DOWN counter corrects the precision with the accuracy equal to or less than the minimum attenuation of the digitally controlled variable attenuator by comparing the level with the set value.

[0016]

(Embodiment 1) FIG. 1 shows Embodiment 1 of an ALC circuit according to the present invention. The ALC circuit includes a digitally controlled variable attenuator 1 for attenuating a pilot signal, a distributor 2 for distributing a pilot signal passing through the digitally controlled variable attenuator 1, and a conversion of the pilot signal distributed by the distributor 2 into a video signal. Linear detector 3, an A / D converter 4 for converting a video signal of the linear detector 3 into a digital signal, a corrector 5 for correcting a digital signal according to frequency and temperature information, and a correction by the corrector 5. A logarithmic converter 6 for converting a digital signal into a logarithmic value, a subtractor 7 for obtaining a difference between the digital signal logarithmically converted by the logarithmic converter 6 and a level setting value, and a digital signal logarithmically converted by the logarithmic converter 6 , A comparator 8 for comparing a level setting value with a level setting value, a level setting device 9 for setting a level setting value, and an output value of a subtractor 7 as an initial value (initial attenuation amount). And, U the output of the comparator 8
An UP / DOWN counter 10 counts up / down as a P / DOWN control signal, and sets the amount of attenuation of the digitally controlled variable attenuator 1 based on the count value.

FIG. 2 shows a level change of a pilot signal according to the first embodiment of the present invention. In FIG. 2, P is an example of a pilot signal, and Pl is an example of a level change of the pilot signal.

Next, the operation of the first embodiment of the present invention will be described.

The pilot signal input to the ALC circuit and passed through the digitally controlled variable attenuator 1 is distributed by the distributor 2, one of which is sent to the next processing circuit, and the other is the video signal by the linear detector 3. Is converted to

After this video signal is converted into a digital signal by the A / D converter 4, the gain fluctuation in the ALC circuit is corrected by the corrector 5 based on the temperature / frequency information.
The corrected signal is converted to a logarithmic value by a logarithmic converter 6, and the signal converted to a logarithmic value is input to a subtractor 7, which calculates a difference between the corrected value and a level set value by a level setter 9. The output value of the subtractor 7 is given to the UP / DOWN counter 10 as an initial value (initial attenuation amount).

The UP / DOWN counter 10 includes a subtractor 7
The attenuation of the digitally controlled variable attenuator 1 is given by the given initial value (initial attenuation).

Next, the pilot signal P whose level has been changed by the amount of attenuation given from the UP / DOWN counter 10 is processed in the same manner as described above, and then input to the comparator 8. The comparator 8 determines the magnitude of the level set value by the level setter 9, and if the level set value is larger than the pilot signal, the UP / DOWN counter 10 is set to "DOWN".
N ”. By this control signal, U
The P / DOWN counter 10 decrements the 1-bit output value. As a result, the digitally controlled variable attenuator 1 reduces the attenuation by the minimum attenuation. These series of processing,
This is repeated while the pilot signal P is being input.

This ALC circuit is provided with a corrector 5 for correcting the digitally converted video signal based on temperature and frequency information, so that the temperature and the temperature in the ALC circuit are corrected.
Frequency fluctuation is corrected, and the set value can be reduced to one.

Further, since the initial attenuation is given by the subtractor 7, and the attenuation of the digitally controlled variable attenuator 1 is adjusted by comparing the level with the level setting value, the attenuation can be corrected with an accuracy equal to or less than the minimum attenuation. it can.

(Embodiment 2) FIG. 3 shows an ALC circuit according to Embodiment 2 of the present invention. The ALC circuit includes a digitally controlled variable attenuator 1 for attenuating a pilot signal, a distributor 2 for distributing a pilot signal passing through the digitally controlled variable attenuator 1, and a conversion of the pilot signal distributed by the distributor 2 into a video signal. Linear detector 3, an A / D converter 4 for converting a video signal of the linear detector 3 into a digital signal, a corrector 5 for correcting a digital signal according to frequency and temperature information, and a correction by the corrector 5. A comparator 8 for comparing the digital signal with the level set value; a level setter 9 for setting the level set value; and a memory for outputting the difference between the digital signal corrected by the corrector 5 and the level set value as a logarithmic value. 13 and an output value of the memory 13 as an initial value (initial attenuation), and an output of the comparator 8 as an up / down control signal as an UP / DOWN control signal. Collected by, it sets the attenuation amount of the digital controlled variable attenuator 1 has a count value UP /
And a DOWN counter 10.

The memory 13 is provided with data for outputting the initial attenuation from the corrected digital signal and the level setting value, and stores the initial attenuation in the UP / DOWN counter 1.
Output to 0. In the first embodiment, the comparison performed by the comparator 8 is a logarithmic value, whereas in the second embodiment, the comparison is performed using an exact value.

Also in this embodiment, the provision of the corrector 5 for correcting the digitally converted video signal based on the temperature / frequency information allows the temperature / frequency in the ALC circuit to be corrected.
Frequency fluctuation is corrected, and the set value can be reduced to one.

Further, since the initial attenuation is given by the memory 13 and then the attenuation of the digitally controlled variable attenuator 1 is adjusted by comparing the level with the level set value, the correction can be made with an accuracy of the minimum attenuation or less. .

(Embodiment 3) FIG. 4 shows an ALC circuit according to Embodiment 3 of the present invention. Embodiment 3 is a modification of Embodiment 1, in which a voltage-controlled variable attenuator 14 is used instead of the digitally-controlled variable attenuator 1, and the output signal (count value) of the UP / DOWN counter 10 is D / The signal is converted into an analog signal (voltage value) by the A converter 15 and supplied to the voltage control variable attenuator 14.

Other than the above, the configuration is the same as that of the first embodiment. Therefore, in FIG. 4, portions corresponding to FIG. 1 are denoted by the same reference numerals as those in FIG. 1, and the description thereof is omitted.

Also in this embodiment, the provision of the corrector 5 for correcting the digitally converted video signal based on the temperature and frequency information provides the temperature and temperature in the ALC circuit.
Frequency fluctuation is corrected, and the set value can be reduced to one.

Further, the initial attenuation is given by the subtractor 7, and the attenuation of the voltage-controlled variable attenuator 14 is adjusted by comparing the level with the level setting value. it can.

(Embodiment 4) FIG. 5 shows an ALC circuit according to Embodiment 4 of the present invention. In the fourth embodiment, a logarithmic detector 16 is used instead of the linear detector 3 in the first embodiment, and the logarithmic converter 6 is omitted accordingly.

The logarithmic detector 16 converts the pilot signal distributed by the distributor 2 into a logarithmic video signal, and performs A / D conversion.
The converter 4 converts the logarithmic video signal into a digital signal.

The corrector 5 corrects the above digital signal in accordance with the frequency and temperature information, and inputs the corrected digital signal to the subtractor 7 and the comparator 8.

Therefore, also in this embodiment, the provision of the corrector 5 for correcting the digitally converted logarithmic video signal based on the temperature / frequency information corrects the temperature / frequency fluctuations in the ALC circuit, and sets the same. The value can be one.

Further, since the initial attenuation is given by the subtractor 7, and the attenuation of the digitally controlled variable attenuator 1 is adjusted by comparing the level with the level setting value, it is possible to correct the attenuation with an accuracy equal to or less than the minimum attenuation. it can.

Embodiment 5 FIG. 6 shows an ALC circuit according to Embodiment 5 of the present invention. In the fifth embodiment, the comparator 8 and the UP / DOWN counter 10 in the first embodiment are omitted, and the output of the subtracter 7 is directly provided to the digitally controlled variable attenuator 1 as the attenuation.

This embodiment is useful in a system in which a setting error of the digitally controlled variable attenuator 1 is allowed and a very high-precision level setting is not required, and the circuit is omitted by omitting the comparator 8 and the UP / DOWN counter 10. The scale can be reduced.

Also in this embodiment, since the corrector 5 for correcting the digitally converted video signal based on the temperature / frequency information is provided, the temperature / frequency fluctuation in the ALC circuit is corrected, and the set value is set to 1 Can be one.

(Embodiment 6) FIG. 7 shows an ALC circuit according to Embodiment 6 of the present invention. The sixth embodiment is an embodiment in which the fifth embodiment in which the comparator 8 and the UP / DOWN counter 10 are omitted is applied to the third embodiment using the voltage-controlled variable attenuator 14.

Therefore, in this embodiment, the output of the subtractor 7 is converted into an analog signal (voltage value) by the D / A converter 15 and is supplied to the voltage-controlled variable attenuator 14.

This embodiment is also useful in a system in which the setting error of the digitally controlled variable attenuator 1 is allowed and a very high-precision level setting is not required, and the circuit is omitted by omitting the comparator 8 and the UP / DOWN counter 10. The scale can be reduced.

Also in this embodiment, since the corrector 5 for correcting the digitally converted video signal based on the temperature and frequency information is provided, the temperature and the temperature in the ALC circuit are corrected.
Frequency fluctuation is corrected, and the set value can be reduced to one.

(Embodiment 7) FIG. 8 shows an ALC circuit according to Embodiment 8 of the present invention. In this embodiment, the subtractor 7, the comparator 8, the U
When the P / DOWN counter 10 is embodied in software by a program executed by the controller 17 and the attenuation amount of the digitally controlled variable attenuator 1 is n,
By performing the processing n times, the level can be set with an accuracy equal to or less than the minimum attenuation of the digitally controlled variable attenuator 1.

The controller 17 controls the digitally controlled variable attenuator 1, and sets the attenuation of the digitally controlled variable attenuator 1 according to the processing flow shown in FIG.

FIG. 9 shows an AL according to the seventh embodiment of the present invention.
FIG. 10 is a processing flowchart of the C circuit, and FIG. 10 shows a level change of a pilot signal when this processing flow is executed.

In the processing flow of FIG. 9, first, the initial attenuation amount N
Is the maximum attenuation M (step S10), and the attenuation ATT
Enter NdB (step S20), and compare the set value with the measured value level (step 30).

If the set value <measured value level, the attenuation A
TT · NdB is kept inserted thereafter (step 4
0). On the other hand, if the set value is not smaller than the measured value level, it is determined whether or not the attenuation ATT · NdB is the minimum attenuation ATT · 0 (step S50). Attenuation ATT / Nd
If B is the minimum attenuation ATT · 0, this processing flow ends, but the attenuation ATT · NdB is the minimum attenuation ATT.
If not 0, N = N-1 (step S60),
Returning to step S20, the attenuation ATT · NdB is reduced and corrected.

In this way, the initial attenuation is given, and the attenuation of the digitally-controlled variable attenuator 1 is adjusted by comparing the level with the level setting value, as in the first embodiment. It can be corrected with accuracy.

Also in this embodiment, since the corrector 5 for correcting the digitally converted video signal based on the temperature and frequency information is provided, the temperature and the temperature in the ALC circuit are corrected.
Frequency fluctuation is corrected, and the set value can be reduced to one.

(Eighth Embodiment) FIG. 11 shows an ALC circuit according to an eighth embodiment of the present invention. This embodiment is a modification of the seventh embodiment, and uses a logarithmic detector 16 instead of the linear detector 3. Except for this point, the configuration is the same as that of the seventh embodiment. Therefore, in FIG. 11, portions corresponding to FIG. 8 are denoted by the same reference numerals as those in FIG. 8, and description thereof is omitted.

Therefore, also in this embodiment, the initial attenuation is given, and then the attenuation of the digitally controlled variable attenuator 1 is adjusted by comparing the level with the level setting value. In addition, since the compensator 5 for correcting the digitally converted video signal based on the temperature and frequency information is provided, temperature and frequency fluctuations in the ALC circuit can be corrected, and the set value can be reduced to one. it can.

[0054]

As can be understood from the above description, according to the ALC circuit according to the present invention, since the corrector for correcting the digitally converted video signal based on the temperature / frequency information is provided, Temperature and frequency fluctuations are corrected, and the set value can be reduced to one.
Further, since the initial attenuation is given by the subtractor, and then the attenuation of the digitally controlled variable attenuator is adjusted by comparing the level with the level setting value, the correction can be performed with an accuracy of the minimum attenuation or less.

According to the ALC circuit of the next invention, since the corrector for correcting the digitally converted video signal based on the temperature / frequency information is provided, the ALC circuit is provided.
Temperature and frequency fluctuations in the circuit are corrected and the set value is set to 1
In addition, the initial attenuation is given by the memory, and the attenuation of the digitally controlled variable attenuator is adjusted by comparing the level with the level setting value. it can.

According to the ALC circuit of the next invention, since the corrector for correcting the digitally converted video signal based on the temperature / frequency information is provided, the ALC circuit is provided.
Temperature and frequency fluctuations in the circuit are corrected and the set value is set to 1
In addition, the initial attenuation is given by a subtractor, and then the attenuation of the voltage-controlled variable attenuator is adjusted by comparing the level with the level setting value. Can be.

According to the ALC circuit of the next invention, since the corrector for correcting the digitally converted logarithmic video signal based on temperature / frequency information is provided,
Temperature and frequency fluctuations in the LC circuit are corrected, the set value can be reduced to one, and the initial attenuation is given by a subtractor, and then the level of the digitally controlled variable attenuator is compared with the level set value. Since the amount of attenuation is adjusted, it can be corrected with an accuracy equal to or less than the minimum amount of attenuation.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an ALC circuit according to a first embodiment of the present invention;

FIG. 2 is a graph showing a change in pilot signal level in the ALC circuit according to the present invention.

FIG. 3 is a block diagram showing an ALC circuit according to a second embodiment of the present invention;

FIG. 4 is a block diagram showing a third embodiment of the ALC circuit according to the present invention;

FIG. 5 is a block diagram showing an ALC circuit according to a fourth embodiment of the present invention;

FIG. 6 is a block diagram showing a fifth embodiment of the ALC circuit according to the present invention;

FIG. 7 is a block diagram showing Embodiment 6 of an ALC circuit according to the present invention.

FIG. 8 is a block diagram showing Embodiment 7 of an ALC circuit according to the present invention.

FIG. 9 is a flowchart showing a processing flow of an ALC circuit according to a seventh embodiment of the present invention.

FIG. 10 is an ALC circuit according to a seventh embodiment of the present invention;
6 is a graph showing a change in pilot signal level at the time of FIG.

FIG. 11 is an eighth embodiment of the ALC circuit according to the present invention;
FIG.

FIG. 12 is a block diagram showing a conventional ALC circuit.

[Explanation of symbols]

1 digitally controlled variable attenuator, 2 distributor, 3 linear detector, 4 A / D converter, 5 corrector, 6 logarithmic converter, 7 subtractor, 8 comparator, 9 level setter,
10 UP / DOWN counter, 13 memory, 14
Voltage controlled variable attenuator, 15 D / A converter, 16 logarithmic detector, 17 controller

Continuation of the front page (56) References JP-A-6-281722 (JP, A) JP-A-5-34442 (JP, A) JP-A-6-242218 (JP, A) JP-A-61-110076 (JP) JP-A-56-18769 (JP, A) JP-A-56-14967 (JP, A) JP-A-2-154179 (JP, A) JP-A-4-93788 (JP, A) 5-505466 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01S ⁷ /00-7/42 G01S 13/00-13/95

Claims (4)

(57) [Claims] 1. A digitally controlled variable attenuating means for attenuating a pilot signal, a distributing means for distributing a pilot signal passed through the digitally controlled variable attenuating means, and a linear for converting the pilot signal distributed by the distributing means into a video signal. Detection means, A / D conversion means for converting a video signal output by the linear detection means into a digital signal, correction means for correcting the digital signal according to frequency and temperature information, and digital signal corrected by the correction means To a logarithmic value, a subtraction unit for calculating a difference between a digital signal logarithmically converted by the logarithmic conversion unit and a set value, and a digital signal and a set value logarithmically converted by the logarithmic conversion unit. Comparing means for comparing, the output of the subtracting means as an initial value and the output of the comparing means An ALC circuit comprising: an UP / DOWN counter that sets an attenuation amount of the digitally controlled variable attenuation means using a count value as an UP / DOWN control signal. 2. A digitally controlled variable attenuating means for attenuating a pilot signal, a distributing means for distributing a pilot signal having passed through the digitally controlled variable attenuating means, and a linear means for converting the pilot signal distributed by the distributing means into a video signal. Detection means, A / D conversion means for converting a video signal output by the linear detection means into a digital signal, correction means for correcting the digital signal according to frequency and temperature information, and digital signal corrected by the correction means Storage means for outputting the difference from the set value as a logarithmic value; comparison means for comparing the digital signal corrected by the correction means with the set value; output of the comparison means using the output of the storage means as an initial value UP / DO
An ALC circuit comprising: an UP / DOWN counter for setting an attenuation amount of the digitally controlled variable attenuation means using a count value as a WN control signal. 3. A voltage-controlled variable attenuating means for attenuating a pilot signal, a distribution means for distributing a pilot signal passed through the voltage-controlled variable attenuating means, and a linear signal for converting the pilot signal distributed by the distribution means into a video signal. Detection means, A / D conversion means for converting a video signal output by the linear detection means into a digital signal, correction means for correcting the digital signal according to frequency and temperature information, and digital signal corrected by the correction means To a logarithmic value, a subtraction unit for calculating a difference between a digital signal logarithmically converted by the logarithmic conversion unit and a set value, and a digital signal and a set value logarithmically converted by the logarithmic conversion unit. Comparing means for comparing the output of the subtracting means as an initial value and the output of the comparing means as UP / DOW An UP / DOWN counter serving as an N control signal; and a D / A converter for converting an output of the UP / DOWN counter into a voltage value and setting an attenuation of the voltage control variable attenuator. ALC circuit. 4. A digitally-controlled variable attenuating means for attenuating a pilot signal, a distribution means for distributing a pilot signal having passed through said digitally-controlled variable attenuating means, and converting the pilot signal distributed by said distribution means into a logarithmic video signal. Logarithmic detection means, A / D conversion means for converting a logarithmic video signal output by the logarithmic detection means into a digital signal, correction means for correcting a digital signal according to frequency and temperature information, and digital correction by the correction means Subtraction means for calculating the difference between the signal and the set value; comparison means for comparing the digital signal corrected by the correction means with the set value; output of the subtraction means being an initial value and output of the comparison means being UP A digital control variable attenuator is set with the count value as a / DOWN control signal. An ALC circuit comprising: an UP / DOWN counter.